1. Field of the Invention
The present invention relates to a porous film and a porous film manufacturing apparatus and, more particularly, to a porous film suitable for a material such as a sanitary material, a medical material, a clothing material, or a wrapping material, and an apparatus for manufacturing the same.
2. Description of the Related Art
Conventionally, as a method of manufacturing a film of this type, a following method is known. In this method, a fine inorganic powder is filled in a general-purpose olefin resin (e.g., polyethylene) in a large amount (generally, 50 vol % or more of the resin) and is formed into a film. The film is then uniaxially or biaxially stretched at a high ratio to form open cells in the interface between the resin and the inorganic powder, thus forming fine pores which communicate with each other in the form of a maze.
In the conventional manufacturing method described above, however, the following problems are posed.
(1) Since a large amount of inorganic powder is added, the essential properties (e.g., strength, softness, and transparency) of a resin constituting a film are considerably deteriorated, and a plastic-like film cannot be substantially obtained.
(2) Since a large amount of inorganic powder is added, and the resultant film is uniaxially or biaxially stretched at a high magnification, this method cannot be applied to a film having elasticity, such as an elastomer film.
(3) since fine pores on the submicron order are formed in the resultant film so as to communicate with each other in the form of a maze, a film has low air permeability although it has vapor permeability. For this reason, practical applications of the film are undesirably limited.
As other porous film manufacturing methods, mechanical perforation methods such as a needle punch method and a heat fusion perforation method are known. In the needle punch method, a thermoplastic resin film is perforated by pressing heated needles against the film. In the heat fusion perforation method, a thermoplastic resin film is perforated by fusing the film by using a heated embossed roll.
A large number of through pores are formed in the porous thermoplastic resin film formed by a mechanical perforation method. For this reason, the size and number of through pores formed in the porous thermoplastic resin film are adjusted to control the gas permeation amount (e.g., the oxygen gas permeation amount). However, water, bacteria, and viruses also permeate through the through pores of the porous thermoplastic resin film. In addition, since the size of each through pore is as large as about 100 .mu.m, it is difficult to highly accurately control the oxygen gas permeation amount.
U.S. Pat. No. 3,718,059 discloses a method of forming fine pores in a thermoplastic resin film comprising the steps of providing a pair of opposite rolls firmly engaged with each other, providing abrasive particles on a surface of one roll, passing the thermoplastic resin film through engagement of the pair of rolls, perforating the film with the abrasive particles by adjusting a pressure and temperature in the engagement of the rolls, forming fine through pores in the film at locations contacting the abrasive particles, and removing the film having the fine pores formed therein from the engagement of the rolls.